1st Instar - Detail
Last night someone emerged from it's egg while we slept. Now 1.5mm long. This is the 1:1 pixel crop. For a little more perspective...
Gender in the monarch larvae: Adult monarch gender is very easy to discriminate, but the larvae are another story. In the pupal stage there is a small visible difference with females having a line dissecting the 8th abdominal sternite. You'll need a dissecting microscope or powerful magnifying glass to find it though. With the larval stages (i.e. caterpillar) ther is no external difference at all. If dissected the male testes can be found as red or pink sac(s) dorsal to the gut in the 6th abdominal segment of late instars.
Hughes, P. R., Radke, C.D., and J. A. A. Renwick. (1993) A Simple, Low-Input Method for Continuous Laboratory Rearing of the Monarch Butterfly (Lepidoptera: Danaidae) for Research. American Entomologist 39: 109-111
More on this butterfly's development at Larval Images